In the related art, as described in Japanese Unexamined Patent Application Publication No. 2008-298821, as an optical element, a diffraction grating is known through which light passes to control the diffraction angle of light. The diffraction grating includes partition walls arranged in parallel at regular intervals. An insulating liquid and a conductive liquid are filled between adjacent partition walls. A voltage is applied to the conductive liquid to change the position of the interface formed between the liquids, such that the grating constant is made variable.
The diffraction grating uses an electrowetting phenomenon (electrocapilliary phenomenon). The electrowetting phenomenon is the phenomenon that, when a voltage is applied to the conductive liquid and the electrode, the surface tension of the liquid is changed and the shape of the liquid surface is thus changed. With the use of this phenomenon, in the diffraction grating, the voltage to be applied increases to lower the surface tension of the liquid and to raise the position of the interface.